The present invention relates to a storage system which includes a disk controller, a disk unit, and an interface unit between the disk controller and the disk unit(s) and, particularly, to a storage system for efficiently controlling many disk units by improving the efficiency for utilizing the interface.
In order to efficiently utilize the disk units, a variety of functions have been specified under the Interface Standards SCSI (small computer system interface) by ANSI (American National Standards Institute). In January, 1994, SCSI-2 was adopted as an ANSI standard. The SCSI-2 specifies a command queuing function according to which a target (disk unit) accepts a plurality of commands from an initiator (disk controller) and queues them. The command queuing is executed in order to reduce the overhead for re-issuing the commands from the disk controller when the disk unit is busy and for reducing the overhead time before the next command processing is started. The queued commands are executed and scheduled so that there is no inconsistency in the commands issued by the disk controller and so that the execution time is at its shortest as indicated by SCSI-2, 7.8.3.2., Example of Tagged Queuing. Queuing the commands as described above allows reduced overhead time and efficient use of the disk unit. The command queuing of SCSI-2 has been disclosed in "Detailed Explanation of SCSI-2 (Latest SCSI Standards and Command Reference)" by Sei-ichi Sugaya, published by CQ Co., pp. 112-119.